
Welcome to the first lecture in 
introductory human physiology. 
And today we want to talk about 
homeostasis, this is the, this is the 
basic theme for physiology. 
All of the organ systems are going to 
integrate in order to maintain 
homeostasis of the body, and the 
homeostasis of the body is to maintain 
conditions within the body that are 
compatible with the life of the cells. 
So, the things that we want to look at 
today, the learning objectives are first 
to explain the basic organization of the 
body, secondly we want to define the 
fluid compartments of the body. 
Third, explain how solutes such as sodium 
chloride, glucose and so forth distribute 
within the body. 
And fourth, we want to explain what 
homeostasis is and the homeostatic 
mechanisms that regulate this, we're 
going to deal with in the very next 
lecture, which is coming right up next. 
And then last, we're going to very 
quickly talk about mass balance and how 
the body maintains mass balance. 
Alright, so the first thing that we 
want to consider then, is the body 
components. 
So, as you all know the body starts with 
this, with, a human body starts with a 
single fertilized egg and this egg then 
undergoes divisions to, to make multiple 
copies as well as differentiation. 
The differentiation allows the specific 
cells to acquire specialized functions. 
These functions then, this groups of 
cells that have the same specialized 
function, will work together to form what 
are called tissues. 
We have four tissue types within the 
body, they are muscle, nervous tissue, 
connective tissue and epithelium. 
These four tissue types will form the 
organs and, and the organs will work 
together as, to, to perform a specific 
function for the body and then, at that 
point if we have more than [INAUDIBLE] 
one more organ functioning. 
That is, we have several organs 
functioning together, then they're called 
an organ system. 
So, for instance an organ system would 
be, an organ would be the kidney and the 
organ system, the renal system or the 
urinary system would be the kidney with 
the two ureters. 
That are taking the urine that's 
generated from the kidneys down to the 
bladder, where the urine can be stored in 
the bladder and then eventually expelled 
to the outside of the body, through what 
is called the urethra. 
So, that's our, our urinary system. 
So, the the organ systems that we're 
going to consider, there are ten organ 
systems in the body, we're going to 
consider nine of them. 
and they are going to perform very 
specific functions. 
So, for instance, the skin, the skin is 
the largest organ of your body. 
It has, its specific function is 
protective, so it forms a barrier of 
tissue to the outside world, it keeps all 
of the inside materials, sort of, 
organized. 
The skin is a, is a very important 
barrier for the loss of water. 
So, it's a hydrophobic barrier, so it 
allows the body to maintain water even 
though we have conditions, where we would 
normally become dehydrated. 
The second of these, of the of the organs 
that we need to deal with and are organs 
that are going to overcome the barrier of 
the skin. 
And that is organs that allow us to have 
entry into the body. 
For instance, the respiratory system 
which allows entry of oxygen and the 
expulsion of CO2. 
So, we have gases then, that can come in 
and out of the body. 
And we have the GI tract or the 
gastrointestinal tract, which allows food 
or nutrients to enter into the body and 
then solid waste to be removed from the 
body. 
We also have transport systems and a 
transport system is predominantly the 
cardiovascular system. 
The cardiovascular system, it takes the 
nutrients that are entering from the GI 
tract and delivers it to all the cells. 
It takes the gases which are coming in 
from the, from the lung the respiratory 
system and delivers that to all of the 
tissues and organs of the body. 
This is done by bulk flow and we'll talk 
about this when we get to the 
cardiovascular system, but this is moving 
materials through a [UNKNOWN] series of 
vessels, which are the vasculature. 
Once we get to the tissues, then we have 
to move the gases and the nutrients and 
the solutes across, out of the 
vasculature and actually, into these 
tissues themselves. 
And they have to [INAUDIBLE] they have to 
cross a very small space and this space 
then is between the tissues of the cells 
and, and the vasculature. 
And we're going to talk about that in 
just a few minutes. 
And that is going to occur by diffusion 
so, that's going to be a very slow 
process that's only a local delivery 
system. 
And then we have to be able to remove 
materials from the body, and this is done 
by the renal system, as I said so, liquid 
wastes are removed excess ions are 
removed. 
Excess water is removed through, through 
from the urinary system. 
And of course the GI tract, the 
gastrointestinal tract will remove waste 
products, solid waste products. 
Alright but the, the physiologist looks 
at the bodies in a slightly different 
manner and that is they divide it into 
what are called fluid compartments. 
We have effectively two major fluid 
compartments, one is where we take all of 
the cytoplasm, that is the liquid 
components that are within cells. 
The cells are bound by a plasma membrane 
and this liquid component, this cytoplasm 
we take all of that from all of the cells 
and, and put it in to one fluid 
compartment. 
And that would be called the 
intracellular fluid compartment or the 
ICF. 
And it is bounded by the plasma membrane, 
and that's what's shown here. 
And then, outside of the cells we have 
this extracellular space and this 
extracellular fluid compartment or the 
ECF, is what is immediately outside of 
all of the cells. 
The ICF, the intracellular fluid 
compartment is the largest off these two 
fluid compartments and it is effectively 
two thirds of the total body water or the 
total fluid of the body. 
And the extra side of food compartment is 
one third now, these two compartments are 
dissimilar in content. 
That is that inside cells, we have very 
high levels of potassium and very small 
concentrations of sodium. 
We also have present within the cells, 
proteins which are negatively charged. 
In the extracellular fluid compartment, 
we have very high concentrations of 
sodium and small concentrations of 
potassium, so we have a completely 
different types of an environment. 
The other thing about these two 
environments is that the extracellular 
fluid compartment can be divided further 
into two compartments. 
One is the intravascular compartment and 
that's within the blood vessels, and the 
other is this interstitial fluid space. 
And this interstitial fluid space is that 
little space that's between the 
vasculature and the cells themselves. 
This is usually filled with connective 
tissue. 
So, these two compartments actually have 
the same content of ions and solutes. 
So, that the amount of sodium that's 
present within the vasculature is equal 
to the amount of sodium that's present 
within the interstitial space. 
And the amount of potassium and so fourth 
is equal between these two compartments. 
So, there is an equilibrium distribute 
distribution, an equal distribution of 
the, of these solutes between the two 
spaces. 
And this is because the, the barrier, 
that is the epithelial cells that are 
lining the blood vessels are [INAUDIBLE] 
a bit leaky. 
And so, they allow this material to move 
from one compartment to the other, and to 
form an equilibrium. 
They, the two compartments do differ in 
that the intravascular fluid compartment 
also has proteins, which are not present 
within the interstitial space. 
Now, the, one other thing about these two 
compartments then, is that we have an 
equilibrium between the intravascular 
space and the interstitial space. 
But we have a disequilibrium between the 
ECF and the ICF. 
And that in, but that is maintained at a 
constant or a steady state and this is 
done so, by the presence of an enzyme 
which is an ATPase, which cleaves ATPs. 
So, the enzyme uses energy to move the 
sodium out of the cells, so 3 Na is 
pumped out of the cells for every 2 K 
that enter the cells. 
This is needed because there are little 
leaks between these two compartments 
would allow potassium then to slowly leak 
out of the cells and into the 
extracellular fluid space. 
And this pump then reorganizes the 
distribution of the ions and keeps the 
ions at an, at a disequilibrium so, that 
we have a steady state, that is, input is 
equal to output. 
But that the amount of sodium on the 
outside of the cells, is different from 
the amount of sodium that's inside the 
cells. 
And the amount of potassium inside the 
cells, is different from the amount of 
potassium that's on the outside of the 
cells. 
So, the fluid compartments then are, the 
total body water is about 60% of your 
total body weight. 
So, if we had an individual who was a 70 
kg male, then 42 L of that individual are 
as fluid, as water. 
That means that the intracellular fluid 
space or the cytoplasm, which is two 
thirds of the total body water, will be 
equal to 28 liters. 
And that the extracellular fluid space, 
which surrounds the cells and is, is 
interface between the cells and the 
external environment, this would be equal 
to 14 liters. 
Then within the, the ECF of the extra 
cellular fluid space we have this 
intravascular fluid and the intravascular 
fluid is actually only one twelfth of the 
total body water. 
And this is one fourths of the ECF, so we 
have one fourths of the ECF is equal to 
the intervascular space times one third, 
which is that which is the ECF. 
That is, of the total body water and that 
gives us then, one twelfth of the total 
body water is equal to the fluid phase of 
the blood, or the vasculature, that is 
equal to the plasma. 
So, that's pretty amazing if you think 
about it because when you think about the 
body, you think of the fluid phase of the 
body is the blood, that is the plasma 
which is the fluid portion, the liquid 
portion of the blood. 
and not all of the other fluids that are 
within the body, but it's actually the 
smallest amount of fluid that's within 
the body. 
So, we have self regulating mechanisms 
then which are active between these 
different fluid phases, these different 
fluid compartments. 
We have an equilibrium, which is allowing 
equal amounts of substance to be 
distributed in between intravascular 
space and the interstitial space. 
So, sodium, potassium, chloride, the 
calcium, they equally distribute between 
these two phases, these two compartments. 
There's no net transfer of substance or 
of energy between these two compartments, 
and there's no barrier to movement. 
As I said, the epithelial cells that are 
lined, that are dividing these two 
compartments are fairly leaky and there's 
no energy expenditure to maintain this 
equilibrium. 
In contrast, we have a steady state which 
is, which is present our extracellular 
fluid space and the intracellular fluid 
space. 
And here, we have a constant amount of 
substance within the compartments. 
And that the input is going to be equal 
to the output. 
But that the, but that the concentrations 
within these two compartments can be 
dissimilar and that this requires energy 
to maintain. 
We need to use ATP, the energy of the 
cells in order to maintain this gradient 
between the two compartments. 
So, why are we so interested in these 
fluid compartments? 
Why is it the physiologists are asking 
about the fluid compartments of the body? 
And the reason for that is that as, that 
the cells themselves require a specific 
factors to be within a very tight range. 
These factors are the amount of oxygen, 
the amount of CO2, the amount of the 
hydrogen ions, the temperature, the 
amount of glucose which is presented to 
the cells. 
So, the, the cells then are requiring 
this very tightly regulated environment 
and yet as you go through your daily 
life, you are bringing into your body a 
very diverse amount of material. 
So, you're constantly changing, your 
environment is constantly changing and it 
is the ECF that is the buffer zone. 
What do I mean about that? 
Well, just think about it, if you eat a 
large hamburger for lunch, you're 
bringing in glucose, fat, proteins, amino 
acids into the body. 
And that material will go from the 
gastrointestinal tract directly into the 
blood and then from the blood, it will 
then be distributed to the cells. 
So, this, but the, but the organs of the 
body are trying to maintain that ECF, 
that buffer zone, which is where all of 
this material is being delivered within a 
normal range, or within a very set range. 
And it's the maintenance of this ECF, the 
compon, this constituents of the ECF as 
relatively constant, which is the main 
theme of physiology and this is what 
homeostasis is about. 
So, that's our central theme, and what 
we're going to see is that all of the 
organs of the body are going to act on 
that ECF to try to keep the contents of 
the ECF under a reg under this very 
narrow range. 
Which is compatible with the life of the 
cells. 
So, what happens if we do not maintain E, 
the ECF in this very tight range of of 
needed factors? 
When we have input is equal to the output 
we'll have wellness. 
So, under those conditions then, as long 
as they, the materials that are within 
the ECF are within the range of that's 
compatible with life of the cells, 
everything is fine. 
But when we have inputs, say for instance 
this is effectively out, is increased 
over output, then we can get illness or 
pathophysiology. 
And the converse can occur, if we have 
output that is greater than input then 
again, we can have illness or 
pathophysiology. 
And so it is this balance, this very 
tight balance that has to be maintained 
at all times in order to keep the body at 
a constant, at a constant activity. 
If the organ system does not perform its 
function, then we can end up with input 
or output which is not equal to, to to 
the opposite. 
Under those conditions then, we will have 
pathophysiology. 
So, one of the major ways that the body 
is going to regulate this, this ECF, is 
by using homeostatic control systems or 
reflex loops. 
And that's what's diagrammed here, and 
that reflex loops have essentially three 
components. 
They have a sensor, which is going to 
detect a specific signal or a stimulus. 
And that sensor then sends the 
information to what is called the 
integration center and this integration 
center is usually the brain. 
The integration center has within it, the 
set points that the bod, that are 
compatible with the life of the cells. 
And so, it will then evaluate the 
incoming signal to see whether or not the 
incoming signal matches the set point 
that the body needs or whether it exceeds 
it or is below it. 
It then will decide whether or not it 
needs to make response, and will send out 
in an affector pathway to [UNKNOWN] , to 
the affectors. 
So, this is an efferent pathway going out 
to the affectors, which will generate a 
response that will then bring the body 
back to a normal, its normal condition. 
This is exactly analogous to the 
temperature control system that you have 
in your house for heating. 
So, the integration center would be our 
rheostat where we set a specific 
temperature that we want within the room. 
And then the stimulus is the, is the 
incoming reading that is, what is the 
temperature of the room. 
And the output would be whether we have 
to turn on the heat, or we have to turn 
on the air conditioning to bring the 
temperature of the room back to normal. 
So, this is a simple reflex loop and it's 
essentially the types of reflex loops 
that the body's going to use. 
So, let's consider one of these systems 
where we have a case where we've decided 
that in a given week, that you want to 
eat nothing but high-salt diet. 
So, on Monday, the amount of sodium 
that's coming into your diet is equal to 
the amount that's being released from the 
body in urine and so, we have then what 
is called a neutral balance. 
So, the mass balance then is equal, 
what's coming to the body is equal to 
what's leaving the body. 
But by Wednesday with this high salt 
diet, you're eating a lot of sodium, 
you're taking in Chinese food with a lot 
of soy sauce on it. 
And so, it's really salty, and so on this 
diet that's very high in salt, we have 
now a positive balance where the amount 
that's coming in from the diet exceeds 
that which is lost in the urine. 
And so, this now is a positive balance 
for sodium. 
But by Friday, now the, the amount of 
sodium that's coming in from the diet is 
equal to the amount that's lost in the 
urine and so we're again under neutral 
balance. 
We're under neutral balance, look at 
what's happened to the body, we've 
actually increased the number of sodium, 
the content of sodium within the body. 
And I just finished telling you that the 
body wants to maintain a very tight 
regulated amount of sodium within the ECF 
at all times. 
That's one of the regulated factors that 
the body is interested in reg, in, in 
keeping constant and yet we have with 
this diet, we have increased the total 
amount of sodium within the body. 
So, how could we do that? 
Well, we've increased the total amount of 
sodium within the body, but what happens 
when you are, are eating in a high salt 
diet? 
What happens when you take in a lot of 
salted food like a potato chips, you eat 
a bag of potato chips what happens? 
You get thirsty and as you get thirsty 
then you get drink water, and as you 
drink water that fluid will come into the 
body and dilute the content of the 
sodium. 
So, that it now has the concentration, 
which is the same as the concentration 
that, of sodium that we had on Monday. 
So, the concentration of the sodium in 
the body is going to stay equal, but the 
content, the amount of sodium that's 
added to the body, has increased. 
And where did it go? 
Well, it went to the ECF, all of the 
sodium went into the ECF. 
It's not able to cross that plasma 
membrane, that hydrophobic barrier. 
And instead is staying in the ECF. 
So, where did the, the volume of water go 
that you drank? 
It also goes into the ECF so, that we 
could dilute then, the sodium 
concentration within the ECF. 
So, all of the volume, all of the fluid 
volume, is into the ECF. 
So, we have increased, the sodium content 
that was in the ECF and we've increased 
the water content, in the ECF. 
But we've maintained, the concentration, 
of sodium, in the ECF, as constant. 
But at what cost? 
So, lets think about it, so what is 
within the ECF? 
We said that there's a vasculature and 
the interstitial fluid space. 
The vasculature, within the vasculature 
we have increased the volume of blood. 
And by increasing the volume of blood, we 
have increased the pressure within the 
vasculature. 
So, by holding this extra sodium and 
holding this extra fluid within the body, 
we increase the volume of, of, of the, of 
the blood, and by doing so, we then 
increase pressure within the 
cardiovascular system. 
So, there was a cost then, to maintaining 
the ECF at a, at a normal, normal range. 
So, what are our general concepts. 
So, the first is the bo, human body then, 
is this interdependent set of self 
regulating systems, whose primary 
function is to maintain an internal 
environment compatible with living cells 
and the tissues. 
And this is homeostasis and this is the 
primary theme of physiology and it is 
what all of the organ systems of the body 
are trying to maintain. 
The second is that we have stability 
these internal variables, and it can be 
achieved by balancing our inputs and our 
outputs to the body and among the organ 
systems. 
But what we need to remember, is that 
there's a hierarchy among the organ 
systems, and that the two organ systems 
that always win out is the brain and the 
heart. 
And often, they will, they will, take 
dominance and allow the body to maintain 
the brain and the heart, say for instance 
profusion of the brain and the heart. 
But then lose the profusion to other 
organ systems. 
So, there is then going to be a trade 
off, where the body is going to make some 
decisions which may not under, under 
difficult conditions or, or pathological 
conditions. 
Which may not maintain everything as a 
constant as a constant. 
Okay so, the next time we come in then, 
let's look at all the different 
mechanisms that we can use to maintain 
this homeostasis. 
Okay, see you then. 

